Regarding the old Warlord comics:
I seem to remember getting those whilst I was in mid-primary school ( mid-'70s ), on account of a friend being from an RAF family, and he was quite into that kind of stuffs.
I remember joining up to the 'club', and getting a little plastic wallet through the post with a top-secret membership card and a code-sheet.
Every week, the comic had a page from the HQ, from the Top Dude who went by the name of Lord Peter Flint, IIRC.
There was always a secret message to be decoded, and it helpfully told you whether it was encoded in 'Letter Code 1', 'Letter Code 2', 'Number Code 1', or Number Code 2'.
Can't remember what any of the messages were, tho!